1. Field of the Invention
This invention relates to pneumatic radial tires, and more particularly to a pneumatic radial tire usable for heavy duty vehicles, particularly airplanes.
2. Related Art Statement
Since radial tires for use in heavy duty vehicles, particularly airplanes are used under considerably peculiar and special conditions as compared with tires used for passenger cars, trucks, buses and the like, research on such tires has been late in starting and these tires are developing at the present. The structure of this tire is not fixed but is roughly divided into the following two parts:
1. As a carcass of radial structure, one or more plies containing organic fiber cords or metal cords, particularly steel cords are used likewise the case for general-purpose vehicles, while plural belt layers each containing one or few organic fiber cords wound at a slight small spiral angle circumferentially of the tread, cords of which layers being crossed with each other, are laminated and used as a belt.
2. The structure of the above item 1 is basic and further a bias cord layer is added as a belt layer. In any case, the tread pattern, tread rubber and the like are substantially the same as usually used in the conventional bias tires for airplanes.
In Japanese Patent laid open No. 61-37,503 is disclosed the improvement of friction performance on snow and ice road surface by producing a difference of hardness in tread rubber between the central portion of the tread and each side portion thereof, the application of which is restricted to general-purpose tires. However, there is no mention of properties of tread rubber in connection with peculiar service environments for airplane radial tires.
When the heavy duty radial tire is run at higher speed under a high internal pressure, and particularly when the shoulder portion of the tread is a so-called round shape as in the airplane tire, tread wearing is accelerated at both side portions of the tread, particularly in the vicinity of each widthwise ground contact end of the tread as compared with the central portion thereof. Consequently, a so-called uneven wearing phenomenon is apt to be caused.
It has been confirmed that the above uneven wear is considerably accelerated when the belt reinforcing the tread has particularly a reinforcing structure hardly producing the deformation based on the rotation of the tire under loading in the vicinity of both side ends of the belt. An example is folded structure of the belt layer at its both side ends or a lamination structure of plural belt layers each containing the cord wound at a slight small spiral angle circumferentially of the tread.
On the other hand, it is indispensable to avoid the occurrence of a so-called standing wave under high-speed running conditions as typified in airplanes. For this purpose, the reinforcing structure as mentioned above is required for enhancing the circumferential stress of the belt. Therefore, the prevention of standing wave is clearly conflicting with the reduction of the uneven wear.